Low Lying Placenta

Low lying placenta is a placenta whose edge lies within 20 mm of the internal cervical os on ultrasound but does not reach or cover it. It is common at the 20 week scan, and most move upward and away as the lower uterine segment grows. It matters because the chance of bleeding is small but real, and the distance of the edge from the os at 36 weeks decides the route of birth.

• Risk factors: previous caesarean section, previous low placenta, multiple pregnancy, age above 35 years, smoking, assisted conception.
• Posterior placentas and those not near a scar resolve more often.
• Most women have no symptoms and a normal outcome.

Most women have no symptoms. Some have light painless bleeding, often after intercourse, with a soft uterus and normal fetal heart.

• No pain and no bleeding in most.
• Abnormal lie in some.
• Differential diagnosis: placenta previa, placental abruption, cervical polyp or ectropion, vasa previa, show in labour.
• Complications: antepartum haemorrhage, haemorrhage in labour, anaemia, preterm birth, malpresentation, postpartum haemorrhage, placenta accreta spectrum with a scar.

The approach is transvaginal ultrasound with measurement of the edge to os distance.

• Transvaginal ultrasound: placental edge less than 20 mm from the internal os without overlap; confirm at 32 weeks and 36 weeks.
• Colour Doppler for accreta spectrum where there is a previous caesarean section.
• Haemoglobin and group in all; cross-match (2 units) at 36 weeks if the edge is within 10 mm.
• No vaginal examination until a scan shows the edge more than 20 mm from the os.

The principle is surveillance and a delivery plan guided by the edge to os distance at 36 weeks.

• Supportive: avoid intercourse if bleeding; avoid strenuous work; teach to attend at once for bleeding or contractions; admit for bleeding; two cannulae in heavy bleeding.
• Pharmacological: ferrous sulphate 200 mg oral 8-hourly for anaemia; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days only if bleeding makes delivery before 35 weeks likely; tranexamic acid 1 g IV over 10 minutes for bleeding; cefazolin 2 g IV before caesarean section; oxytocin 10 IU IM at birth then infusion of 20 IU in 1 litre sodium chloride 0.9 percent at 125 mL per hour for 4 hours; carboprost 250 micrograms IM every 15 minutes (maximum 8 doses) for atony, avoided in asthma.
• Procedures: repeat scan at 32 and 36 weeks; vaginal birth with senior review and caesarean theatre ready if the edge is 11 to 20 mm from the os; caesarean section at 37 to 38 weeks and 6 days if the edge is 0 to 10 mm from the os; urgent caesarean section for heavy bleeding or fetal compromise.
• Special groups: RhD negative women give anti-D immunoglobulin 500 IU IM within 72 hours of each bleed; previous caesarean section with suspected placenta accreta spectrum needs birth in a tertiary unit.
• Red flags: bleeding soaking more than 2 pads in an hour, contractions with bleeding, shock, abnormal fetal heart rate.
• Follow-up: ultrasound at 32 and 36 weeks; haemoglobin every 4 weeks; antenatal review every 2 to 4 weeks; postnatal haemoglobin and iron.